Recently, a two-terminal organic luminescent device has been developed as a self-emission device, and thus an OELD panel or an OELD device employing the two-terminal organic luminescent device has been actively studied.
The two-terminal organic luminescent device is typically referred to as an organic light emitting diode (OLED). The OLED has a pair of electrodes, that is a cathode and an anode, and an organic light-emitting layer interposed between the cathode and the anode. The representative OLED has a structure of anode/hole transporting layer/organic light-emitting layer/electron transporting layer/cathode, which was suggested by Tang et al. The OLED with such a multi-layered structure has high luminescent efficiency and thus has been adopted in almost all kinds of current OELDs under the development. Further, a hole injecting layer may be formed between the anode and the hole transporting layer or an electron injecting layer may be formed between the electron transporting layer and the cathode.
When a voltage is applied to the OELD, the electrons from the cathode and the holes from the anode are injected into the organic light-emitting layer, and thereafter the recombination of the injected holes and electrons at the organic light-emitting layer excites the emitting centers, thereby emitting electro-luminescent (EL) lights. In the present specification, all layers interposed between the cathode and the anode are referred to as an “EL layer”. Therefore, the EL layer can include the above-mentioned hole injecting layer, hole transporting layer, light-emitting layer, electron transporting layer and electron injecting layer. A single organic layer can also be used to form the EL layer. Also, luminescence from the EL device in the present specification is referred to as a “driving of the EL device”.
Driving system of the OELD includes a passive matrix type system and an active matrix type system. The passive matrix type OELD has an anode of transparent electrode formed on a transparent insulating substrate in the form of stripe patterns, an organic EL layer and a cathode of metal in the form of stripe patterns perpendicular to the anode, which are sequentially formed.
In the passive matrix type OELD, one of scanning lines of cathode (or anode) is selected and one or more data signal lines of anode (or cathode) among plurality of data signal lines that are connected to the selected scanning line are selected. Thus, luminescence is produced from the pixels connected to the selected scanning line and the selected data signal lines. Date signal (or video signal) inputted from an outside is edited by a signal driving circuit, thereby being produced as a signal line. Integrated circuit (IC) Chip for the signal line driving circuit may be mounted on a display in a manner of tape automated bonding (TAB) or may be directly bonded to a pixel substrate, thereby being assembled with the display.
The active matrix type OELD includes a driving semiconductor element and a switching semiconductor element which are prepared for each of a plurality of pixels arranged in the form of matrix type. The semiconductor elements are connected to one of the cathode and the anode of the OELD, thereby controlling current of the OELD. Unit pixel of the active matrix type OELD includes an OLED and a transistor (or transistors) having a semiconductor film formed on an insulating substrate. Here, the transistor having a semiconductor film is hereinafter referred to as “a thin film transistor (TFT)”.
In detail, the active matrix type OELD has a plurality of OLEDs, a plurality of gate signal lines, a plurality of source signal lines, a plurality of power supply lines, a plurality of TFTs for driving and switching and a plurality of capacitors, which are formed on the insulating substrate. Generally, at least two TFTs including at least one switching TFT and at least one driving TFT and one capacitor are provided for each of pixels in the active matrix type OELD.
Recently considerable progress has been made in manufacturing the TFT on the substrate. In particular, a TFT made of poly silicon has a higher mobility than that of a conventional TFT made of amorphous silicon and thus can be operated faster. In addition, each pixel was controlled by an external driving circuit. But the poly-silicon TFT technology is capable of directly forming a driving circuit for controlling pixel on the substrate.
As described above, since various driving circuit and elements of the active matrix type OELD can be formed on single substrate, the active matrix type OELD has many advantages such as cost-down, a yield-enhancement and a display-miniature. Also, the OLED does not require a backlight source, so a thickness and a weight of the display using the OLED can be relatively reduced in comparison with a liquid crystal display (LCD). By the above-mentioned reasons, it is a trend that the OELD is used as a display section of a portable information terminal such as a mobile computer, a mobile telephone, a mobile game player or an electronic book etc., instead of the LCD. The development of the OELD having a higher brightness of luminance under a lower operating voltage to accommodate the present trend, have been continuously performed.